Pain often develops from diseases that affect the somatosensory system. One disease that is often implicated is diabetes mellitus. Diabetes may affect the nervous system in different ways but one of the classical disorders is a length dependent neuropathy. Here axons with longer axons preferentially are involved in a neuropathy which is associated with both degeneration and a sensitization of nociceptors. The classic feature is burning pain typically involving the feet given that the axons to the feet represent the longest primary afferents in the body. This problem may occur early or late in the disease, and in fact may occur in so-called pre-diabetes which is a condition representing a disorder of glucose metabolism without strictly meeting the criteria for diabetes mellitus. It is now appreciated that diabetes is but one cause of a length dependent neuropathy. The painful symptoms that accompany these disorders, including an idiopathic small fiber neuropathy, are nearly identical with that seen in diabetes mellitus. Treatments directed at treatment of the diabetes mellitus itself may help slow the progression of the neuropathy but do not necessarily address the pain. There are no known treatments for idiopathic length dependent small fiber neuropathy. Certain chemotherapeutic drugs induce a length dependent neuropathy associated with pain. This pain may limit dosing and thus affect the adequacy of the cancer treatment. Systemic treatments of pain include use of opioids, anticonvulsants, antidepressants, and membrane stabilizers. All of these therapies are frequently ineffective and typically their use is accompanied by a substantial adverse side effect profile. Systemic therapies can be given by the oral route, or by patches applied to the skin. Lidocaine patches can be applied to the skin. Their value in treatment of pain associated with length dependent neuropathies is limited because of numbing of the skin. Capsaicin can be applied locally to the skin but application is associated with significant pain and the capsaicin destroys nociceptor function.
Some prior attempts also have been made to treat painful diabetic neuropathy with clonidine, a potent α2-adrenergic partial agonist used primarily for the treatment of hypertension (Jarrott et al., “Clonidine: Understanding its disposition, sites, and mechanism of action”, Clin. Exp. Pharm. Physiol., 14, 471-479 (1987)). Clonidine has been applied topically to areas remote to the painful area as an alternative to oral delivery for effecting systemic delivery. For example, in a placebo-controlled cross-over pain trial in patients with painful diabetic neuropathy, no statistically significant difference between patients receiving systemic clonidine administered with transdermal patches and patients receiving placebo patches was observed (Zeigler et al. Pain 48: 403-408 (1992)). In a follow-up placebo controlled pain study in similar patients with painful diabetic neuropathy, transdermal patches delivering systemic levels of clonidine were evaluated using a two-stage enriched enrollment design (Byas-Smith et al. Pain 60: 267-274 (1995)). Only twelve of forty-one patients (29%) who completed the initial course of treatment were considered clonidine responders. These twelve clonidine responders were then rechallenged in a second placebo controlled study which used the highest dosage available with the transdermal patch system. The pain reduction relative to placebo tended to be modest although statistically significant (p<0.015). The site of action of clonidine was not studied in this study. In principal the site of action could be central or peripheral. In other pain conditions a central analgesic action of clonidine has been determined. This treatment involved systemic delivery of clonidine with a transdermal patch applied remote to the painful area, resulting in systemic blood levels exceeding 0.2 ng/ml.
It is therefore an object of the present invention to provide methods and compositions to effectively treat or alleviate pain in length dependent or other neuropathies, as may be associated with diabetes, by topical local delivery to the painful area of an alpha-2 adrenergic agonist.